High-Performance Trimethylamine Sensor Based on an Imine Covalent Organic Framework

As trimethylamine (TMA) is widely used in agriculture and industry, inhalation of TMA can cause very serious negative effects on human health. However, most of the current gas sensors for detecting TMA are commonly performed at high temperatures and cannot meet market needs. Inspired by this, we prepared imine covalent organic frameworks (TB-COF) synthesized from two monomers, 1,3,5-tris(4-aminophenyl)benzene (TAPB) and 1,3,5-benzotricarboxaldehyde (BTCA), using acetic acid as a catalyst at room temperature. Based on this, three sensors were prepared for gas sensitivity testing, namely, TA, BT, and TB-COF sensors. The three sensors were tested for 15 different gases at room temperature. From the whole gas sensitivity data, the TB-COF sensor made by compositing TA and BT has a higher sensitivity (6845.9%) to TMA at 500 ppm, which is 6.1 and 5.4 times higher than the response of TA and BT sensors, respectively. The TB-COF sensor adsorbs and desorbs TMA in a controlled 23 s cycle with a low detection limit of 28.6 ppb. This result indicates that TB-COF prepared at room temperature can be used as a gas-sensitive sensing material for real-time monitoring of TMA. The gas sensing results demonstrate the great potential of COFs for sensor development and application and provide ideas for further development of COFs-based gas sensors.

Ambipolar surface conduction in oxygen sub-stoichiometric molybdenum oxide films

The surface electric conduction in amorphous and crystallized molybdenum oxide films was studied as a function of electronic structure by current-voltage and simultaneous spectroscopic ellipsometry measurements on structures of the kind Al/Molybdenum oxide (MoOx)/Al, at temperatures up to 400 °C and in ambient air. At room temperature, both amorphous and crystalline MoOx samples were found to be sub-stoichiometric in oxygen. The random distribution of oxygen vacancies and the imperfect atomic ordering induced the creation of an intermediate band (IB) located near the valence band and of individual electronic gap states. At temperatures below 300 °C, the conduction was found to exhibit ambipolar character in which electrons and holes participated, the former moving in the conduction band and the latter in the IB and though gap states. Above 300 °C, due to samples gradual oxidation and improvement of atomic ordering (samples crystallization), the density of states in the IB and the gap gradually decreased. The above in their turn resulted in the gradual suppression of the ambipolar character of the conduction, which at 400 °C was completely suppressed and became similar to that of ordinary n-type semiconductor. The above phenomena were found to be reversible, so as the semiconducting MoOx samples were returning to room temperature the ambipolarity of the conduction was gradually re-appearing giving rise to an unusual phenomenon of "metallic" temperature variation of electrical resistance when electrons were injected.

MoO3 Nanorods Decorated by PbMoO4 Nanoparticles for Enhanced Trimethylamine Sensing Performances at Low Working Temperature

The gas sensing performance of metal oxides is limited by the lack of conductivity and sensing activity. Inducing the release of more electrons and activating more chemisorbed oxygen ions to participate in the gas sensing reaction can effectively overcome this limitation. The development of a PbMoO₄/MoO₃ heterostructure prepared by the addition of Pb²⁺ ions with MoO₃ nanorods is reported for highly sensitive and selective trimethylamine (TMA) detection. The response of the PbMoO₄/MoO₃ sensor (33.2) to 10 ppm TMA is improved 3-fold compared to the MoO₃ sensor (10.7), and the working temperature is reduced from 170 to 133 °C. The enhanced gas sensing performance and mechanism of PbMoO₄/MoO₃ were demonstrated using the energy band diagram and X-ray photoelectron spectroscopy (XPS) analysis. It is mainly attributed to the following promotion: (1) the induction of Pb²⁺ ions increases the electron density around the Mo element, enabling the decorated MoO₃ to release electrons easily; (2) the formed PbMoO₄/MoO₃ heterojunction endows a high degree of electron transfer at the interface; (3) the formation of the potential barrier causes the device resistance to decrease significantly upon TMA exposure. Finally, the practicability of the sensor was verified by detecting TMA released from Carassius auratus and shrimp to reflect their freshness.

Rapid synthesis of vertically aligned α-MoO3 nanostructures on substrates

We report a new procedure for large scale, reproducible and fast synthesis of polycrystalline, dense, vertically aligned α-MoO₃ nanostructures on conducting (FTO) and non-conducting substrates (Si/SiO₂) by using a simple, low-cost hydrothermal technique. The synthesis method consists of two steps, firstly formation of a thermally evaporated Cr/MoO₃ seed layer, and secondly growth of the nanostructures in a highly acidic precursor solution. In this report, we document a growth process of vertically aligned α-MoO₃ nanostructures with varying growth parameters, such as pH and precursor concentration influencing the resulting structure. Vertically aligned MoO₃ nanostructures are valuable for different applications such as electrode material for organic and dye-sensitized solar cells, as a photocatalyst, and in Li-ion batteries, display devices and memory devices due to their high surface area.

We report a procedure for large scale, reproducible and fast synthesis of polycrystalline, dense, vertically aligned α-MoO₃ nanostructures on conducting (FTO) and non-conducting substrates (Si/SiO₂) by using a simple, low-cost hydrothermal technique.

Fabrication of Biobased Hydrophobic Hybrid Cotton Fabrics Using Molecular Self-Assembly: Applications in the Development of Gas Sensor Fabrics

Inadvertent inhalation of various volatile organic compounds during industrial processes, such as coal and metal mining, metal manufacturing, paper and pulp industry, food processing, petroleum refining, and concrete and chemical industries, has caused an adverse effect on human health. In particular, exposure to trimethylamine (TMA), a fishy odor poisonous gas, resulted in numerous health hazards such as neurotoxicity, irritation in eyes, nose, skin, and throat, blurred vision, and many more. According to the environmental protection agency, TMA in the level of 0.10 ppm is generally considered as safe, and excess dose results in "trimethylaminuria" or "fish odor syndrome." In order to avoid the health hazards associated with the inhalation of TMA, there is an urge to design a sensor for TMA detection even at low levels for use in food-processing industries, medical diagnosis, and environment. In this report, for the first time, we have developed a TMA sensor fabric using a sequential self-assembly process from silver-incorporated glycolipids. Formation of self-assembled supramolecular architecture, interaction of the assembled structure with the cotton fabric, and sensing mechanism were completely investigated with the help of various instrumental methods. To our surprise, the developed fabric displayed a transient response for 1-500 ppm of TMA and a stable response toward 100 ppm of TMA for 15 days. We believe that the reported flexible TMA sensor fabrics developed via the sequential self-assembly process hold great promise for various innovative applications in environment, healthcare, medicine, and biology.

Dual Selective Gas Sensing Characteristics of 2D α-MoO3-x via a Facile Transfer Process

Metal oxide-based gas sensor technology is promising due to their practical applications in toxic and hazardous gas detection. Orthorhombic α-MoO₃ is a planar metal oxide with a unique layered structure, which can be obtained in a two-dimensional (2D) form. In the 2D form, the larger surface area-to-volume ratio of the material facilitates significantly higher interaction with gas molecules while exhibiting exceptional transport properties. The presence of oxygen vacancies results in nonstoichiometric MoO₃ (MoO_3-x), which further enhances the charge carrier mobility. Here, we study dual gas sensing characteristics and mechanism of 2D α-MoO_3-x. Herein, conductometric dual gas sensors based on chemical vapor deposited 2D α-MoO_3-x are developed and demonstrated. A facile transfer process is established to integrate the material into any arbitrary substrate. The sensors show high selectivity toward NO₂ and H₂S gases with response and recovery rates of 295.0 and 276.0 kΩ/s toward NO₂ and 28.5 and 48.0 kΩ/s toward H₂S, respectively. These gas sensors also show excellent cyclic endurance with a variation in ΔR ∼ 112 ± 1.64 and 19.5 ± 1.13 MΩ for NO₂ and H₂S, respectively. As such, this work presents the viability of planar 2D α-MoO_3-x as a dual selective gas sensor.

Selective Detection of Nitrogen-Containing Compound Gases

N-containing gaseous compounds, such as trimethylamine (TMA), triethylamine (TEA), ammonia (NH₃), nitrogen monoxide (NO), and nitrogen dioxide (NO₂) exude irritating odors and are harmful to the human respiratory system at high concentrations. In this study, we investigated the sensing responses of five sensor materials—Al-doped ZnO (AZO) nanoparticles (NPs), Pt-loaded AZO NPs, a Pt-loaded WO₃ (Pt-WO₃) thin film, an Au-loaded WO₃ (Au-WO₃) thin film, and N-doped graphene—to the five aforementioned gases at a concentration of 10 parts per million (ppm). The ZnO- and WO₃-based materials exhibited n-type semiconducting behavior, and their responses to tertiary amines were significantly higher than those of nitric oxides. The N-doped graphene exhibited p-type semiconducting behavior and responded only to nitric oxides. The Au- and Pt-WO₃ thin films exhibited extremely high responses of approximately 100,000 for 10 ppm of triethylamine (TEA) and approximately −2700 for 10 ppm of NO₂, respectively. These sensing responses are superior to those of previously reported sensors based on semiconducting metal oxides. On the basis of the sensing response results, we drew radar plots, which indicated that selective pattern recognition could be achieved by using the five sensing materials together. Thus, we demonstrated the possibility to distinguish each type of gas by applying the patterns to recognition techniques.

Enhanced Gas-Sensing Properties for Trimethylamine at Low Temperature Based on MoO3/Bi2Mo3O12 Hollow Microspheres

Most reported trimethylamine (TMA) sensors have to operate at high temperature, which will consume energy highly. To detect TMA at low temperature, it is necessary to modify the existing materials or develop new materials. In this paper, the sensor based on MoO₃/Bi₂Mo₃O₁₂ hollow microspheres can work at low operating temperature of 170 °C, which were prepared via a simple solvothermal route. The phase and morphology of the product were characterized by an X-ray diffraction meter, a scanning electron microscope and a transmission electron microscope. The surface chemistry of the MoO₃/Bi₂Mo₃O₁₂ sensor was studied with an X-ray photoelectron spectroscope to investigate the TMA sensing mechanism. The MoO₃/Bi₂Mo₃O₁₂ sensor ( S = 25.8) had a higher response to 50 ppm TMA than those of MoO₃ hollow spheres ( S = 10.8) and Bi₂Mo₃O₁₂ sensors ( S = 4.8) at 170 °C. In contrast to the pure MoO₃ and Bi₂Mo₃O₁₂ sensors, the MoO₃/Bi₂Mo₃O₁₂ sensor exhibited an obviously enhanced gas-sensing property for TMA, which might be due to the heterostructure formed between MoO₃ and Bi₂Mo₃O₁₂ and the hollow morphology. It is the first time for MoO₃/Bi₂Mo₃O₁₂ to apply in gas sensors, which might take an important step in the application of MoO₃/Bi₂Mo₃O₁₂ or Bi₂Mo₃O₁₂ in the field of gas sensing.

Electrically-Transduced Chemical Sensors Based on Two-Dimensional Nanomaterials

Electrically-transduced sensors, with their simplicity and compatibility with standard electronic technologies, produce signals that can be efficiently acquired, processed, stored, and analyzed. Two dimensional (2D) nanomaterials, including graphene, phosphorene (BP), transition metal dichalcogenides (TMDCs), and others, have proven to be attractive for the fabrication of high-performance electrically-transduced chemical sensors due to their remarkable electronic and physical properties originating from their 2D structure. This review highlights the advances in electrically-transduced chemical sensing that rely on 2D materials. The structural components of such sensors are described, and the underlying operating principles for different types of architectures are discussed. The structural features, electronic properties, and surface chemistry of 2D nanostructures that dictate their sensing performance are reviewed. Key advances in the application of 2D materials, from both a historical and analytical perspective, are summarized for four different groups of analytes: gases, volatile compounds, ions, and biomolecules. The sensing performance is discussed in the context of the molecular design, structure-property relationships, and device fabrication technology. The outlook of challenges and opportunities for 2D nanomaterials for the future development of electrically-transduced sensors is also presented.

High-Performance Flexible ZnO Nanorod UV/Gas Dual Sensors Using Ag Nanoparticle Templates

Flexible zinc oxide (ZnO) nanorod (NR) ultraviolet (UV)/gas dual sensors using silver (Ag) nanoparticle (NP) templates were successfully fabricated on a polyimide substrate with nickel electrodes. Arrays of Ag NPs were used as a template for the growth of ZnO NRs, which could enhance the flexibility and the sensing properties of the devices through the localized surface plasmon resonance (LSPR) effect. The Ag NPs were fabricated by the rapid thermal annealing process of Ag thin films, and ZnO NRs were grown on Ag NPs to maximize the surface area and form networks with rod-to-rod contacts. Because of the LSPR effect by Ag NPs, the UV photoresponse of the ZnO NRs was amplified and the depletion region of ZnO NRs was formed quickly because of the Schottky contact with the Ag NPs. As a consequence, ZnO NR UV/gas dual sensors grown on the Ag NP template with a diameter of 28 nm exhibited the outstanding UV-sensing characteristics with a UV on-off ratio of 3628 and a rising time ( t_r) and a decay time ( t_d) of 3.52 and 0.33 s upon UV exposure, along with excellent NO₂-sensing characteristics with a stable gas on-off ratio of 288.5 and a t_r and t_d of 38 and 62 s upon NO₂ exposure. Furthermore, the sensors grown on the Ag NP template exhibited good mechanical flexibility and stable sensing properties without significant degradation even after the bending test up to 10 000 cycles at the bending radius of 5 mm.

Synergy between nanomaterials and volatile organic compounds for non-invasive medical evaluation

This article is an overview of the present and ongoing developments in the field of nanomaterial-based sensors for enabling fast, relatively inexpensive and minimally (or non-) invasive diagnostics of health conditions with follow-up by detecting volatile organic compounds (VOCs) excreted from one or combination of human body fluids and tissues (e.g., blood, urine, breath, skin). Part of the review provides a didactic examination of the concepts and approaches related to emerging sensing materials and transduction techniques linked with the VOC-based non-invasive medical evaluations. We also present and discuss diverse characteristics of these innovative sensors, such as their mode of operation, sensitivity, selectivity and response time, as well as the major approaches proposed for enhancing their ability as hybrid sensors to afford multidimensional sensing and information-based sensing. The other parts of the review give an updated compilation of the past and currently available VOC-based sensors for disease diagnostics. This compilation summarizes all VOCs identified in relation to sickness and sampling origin that links these data with advanced nanomaterial-based sensing technologies. Both strength and pitfalls are discussed and criticized, particularly from the perspective of the information and communication era. Further ideas regarding improvement of sensors, sensor arrays, sensing devices and the proposed workflow are also included.

Facile Water-Based Strategy for Synthesizing MoO3-x Nanosheets: Efficient Visible Light Photocatalysts for Dye Degradation

Nanostructured molybdenum oxides are promising materials for energy storage, catalysis, and electronic-based applications. Herein, we report the synthesis of MoO_3-x nanosheets (x stands for oxygen vacancy) via an environmentally friendly liquid exfoliation approach. The process involves the reflux of the bulk α-MoO₃ precursor in water at 80 °C for 7 days. Electron microscopy and atomic force microscopy show that the MoO_3-x nanosheets are a few nanometer thick. MoO_3-x nanosheets exhibit near infrared plasmonic property that can be enhanced by visible light irradiation for a short time (10 min). Photocatalytic activity of MoO_3-x nanosheets for organic dye decolorization is examined using two different dyes (rhodamine B and methylene blue). Under visible light irradiation, MoO_3-x nanosheets make a rapid decolorization for the dye molecules in less than 10 min. The simple synthesis procedure of MoO_3-x nanosheets combined with their remarkable photochemical properties reflect the high potential for using the nanosheets in a variety of applications.

Toward breath analysis on a chip for disease diagnosis using semiconductor-based chemiresistors: recent progress and future perspectives

Semiconductor gas sensors using metal oxides, carbon nanotubes, graphene-based materials, and metal chalcogenides have been reviewed from the viewpoint of the sensitive, selective, and reliable detection of exhaled biomarker gases, and perspectives/strategies to realize breath analysis on a chip for disease diagnosis are discussed based on the concurrent design of high-performance sensing materials and miniaturized pretreatment components. Carbon-based sensing materials that show relatively high responses to NO and NH₃ at low or mildly raised temperatures can be applied to the diagnosis of asthma and renal disease. Halitosis can be diagnosed by employing sensing or additive materials such as CuO and Mo that have high chemical affinities for H₂S, while catalyst-loaded metal oxide nanostructure sensors or their arrays have been used to diagnose diabetes via the selective detection of acetone or by pattern recognition of sensor signals. For the ultimate miniaturization of a breath-analysis system into a tiny chip, preconditioning that includes preconcentration, dehumidification, and flow sensing needs to be either improved through the design of gas/moisture adsorbents or removed/simplified through the design of highly sensitive sensing materials that are less impervious to interference from humidity and temperature. Moreover, an abundant sensing library needs to be provided for the diagnosis of diseases (e.g. lung cancer) that are associated with multiple biomarker gases and for finding new methods to diagnose other diseases. For this aim, p-type oxide semiconductors with high catalytic activities, as well as combinatorial approaches, can be considered for the development of sensing materials that detect less-reactive large molecules, and high-throughput screening, respectively. Selectivity for a specific biomarker gas will simplify the system further. Breath analysis on a tiny chip using semiconductor chemiresistors with ultralow power consumption that is connected to the 'Internet of Things' will pave new roads for disease diagnosis and patient monitoring.

Micropatternable Double-Faced ZnO Nanoflowers for Flexible Gas Sensor

Micropatternable double-faced (DF) zinc oxide (ZnO) nanoflowers (NFs) for flexible gas sensors have been successfully fabricated on a polyimide (PI) substrate with single-walled carbon nanotubes (SWCNTs) as electrode. The fabricated sensor comprises ZnO nanoshells laid out on a PI substrate at regular intervals, on which ZnO nanorods (NRs) were grown in- and outside the shells to maximize the surface area and form a connected network. This three-dimensional network structure possesses multiple gas diffusion channels and the micropatterned island structure allows the stability of the flexible devices to be enhanced by dispersing the strain into the empty spaces of the substrate. Moreover, the micropatterning technique on a flexible substrate enables highly integrated nanodevices to be fabricated. The SWCNTs were chosen as the electrode for their flexibility and the Schottky barrier they form with ZnO, improving the sensing performance. The devices exhibited high selectivity toward NO₂ as well as outstanding sensing characteristics with a stable response of 218.1, fast rising and decay times of 25.0 and 14.1 s, respectively, and percent recovery greater than 98% upon NO₂ exposure. The superior sensing properties arose from a combination of high surface area, numerous active junction points, donor point defects in the ZnO NRs, and the use of the SWCNT electrode. Furthermore, the DF-ZnO NF gas sensor showed sustainable mechanical stability. Despite the physical degradation observed, the devices still demonstrated outstanding sensing characteristics after 10 000 bending cycles at a curvature radius of 5 mm.

Hierarchical and Hollow Fe2O3 Nanoboxes Derived from Metal-Organic Frameworks with Excellent Sensitivity to H2S

Hierarchical and hollow porous Fe₂O₃ nanoboxes (with an average edge length of ∼500 nm) were derived from metal-organic frameworks (MOFs) and the gas sensing characteristics were investigated. Sensors based on Fe₂O₃ nanoboxes exhibited a response (resistance ratio) of 1.23 to 0.25 ppm (ppm) hydrogen sulfide (H₂S) at 200 °C, the response/recovery speed is fast and the selectivity to H₂S is excellent. Remarkably, the sensor showed fully reversible response to 5 ppm of H₂S at 50 °C, demonstrating its promise for operating at near room temperature, which is favorable for medical diagnosis and indoor/outdoor environment monitoring. The excellent performance of the Fe₂O₃ nanoboxes can be ascribed to the unique morphology with high specific surface area (SSA) and porous nanostructure.